Current research into electrochemical supercapacitors (also referred to as ultracapacitors or electric double layer capacitors (“EDLCs”)), has revealed that these devices may be promising local energy storage devices. Other available energy storage technologies such as the Faradaic battery and conventional dielectric capacitors have drawbacks. Batteries are characterized by high energy density, low power density, and short cycle life, while dielectric capacitors are low energy density, high power density and have a long cycle life. In contrast, supercapacitors potentially may be characterized by mid-range energy storage capability, high power density and long cycle life.
Three general types of supercapacitors may be identified, such as: (1) carbon-based active materials that store charge via high surface area; (2) oxidation-reduction (“redox” or pseudo-capacitors) which use fast and reversible surface or near surface reactions for charge storage (such as via transition metal oxides or conducting organic polymers); and (3) hybrid capacitors that combine capacitive or pseudo-capacitive electrodes with battery electrodes.
Carbon-based high surface area supercapacitors store electricity by physical charge separation. Supercapacitor charge is stored through reversible ion adsorption on high surface area electrodes. Carbon nanotubes (“CNTs”) (such as single-walled carbon nanotubes (“SWCNTs”) and multi-walled carbon nanotubes (“MWCNTs”)) and carbon fibers have been explored for use in the electrodes of such carbon-based, high surface area supercapacitors, typically forming an intertwined, matted or entangled mesh layer limiting the available surface area for ion adsorption on the exterior of the fibers or nanotubes, and further having a very large variation in pore size.
Other attempts to use carbon nanotubes have included use of aligned CNTs having either a closed (capped) or an open (or uncapped) end, but have proved extraordinarily difficult to manufacture and scale up to commercial quantities. For example, capped CNTs have been grown directly on electrode plates to achieve some alignment; in another instance, carbon nanotubes have been aligned by growth of the CNTs through vacuum chemical vapor deposition on a silicon wafer substrate, then metallized and transferred by hand to an electrode using a double-sided conducting tape, followed by plasma etching to uncap and open the ends and etch mesopores in the CNTs. While theoretically feasible, such alignment methods based on the growth of the CNTs are not practical beyond a laboratory environment. Further, such complicated CNT and capacitor fabrication processes are prohibitively expensive, are not scalable and have not been able to achieve commercial production. Such capacitor structures have not fully exploited the interior surfaces of the CNTs and the potential pore sizes of CNTs, have not addressed other methods of producing CNTs and the post-growth alignment of CNTs, and have not addressed specific energy density limitations of the resulting capacitors.
Accordingly, a need remains for methods of manufacturing a CNT-based capacitor using readily available commercial methods for fabricating CNTs and using post-growth alignment of the CNTs. Such a CNT-based capacitor should exploit the interior surfaces of the CNTs and the potential pore sizes of CNTs. Such a CNT-based capacitor should be capable of being manufactured at a commercial scale and comparatively low cost, while simultaneously providing comparatively high power density, high energy density, and long cycle life.